The study proposes to test the following hypotheses: 1) Heart rate buffers changes in arterial pressure through changes in cardiac output 2) The high mean heart rates which occur with autonomic blockade influence blood pressure regulation 3) The effect of central volume on arterial pressure is mediated either entirely through the mechanical influence on central volume, or in part through a cardiopulmonary reflex mediated change in arterial vascular resistance. A complete understanding of the mechanisms by which central volume influences arterial pressure in the intact human circulation has implications for the study of normal physiology, and more importantly, the regulation of arterial pressure in settings such as the intensive care unit, where autonomic control is often disrupted and centeral volume may be controlled exogenously.